Describe how tectonic plates move and explain what happens at divergent, convergent and transform plate boundaries

What this dot point is asking

This outcome asks you to explain how the plates move and to describe what happens at the three types of plate boundary, including the landforms and hazards each produces. Boundaries are where almost all the world's earthquakes and volcanoes occur, so this is central to the topic. The central idea is that the way two plates meet, moving apart, together or past each other, decides what happens at the boundary.

The answer

How plates move

The crust is broken into large pieces called tectonic plates that float on the semi-molten mantle. Convection currents in the mantle, driven by the Earth's internal heat, drag the plates very slowly, only a few centimetres a year. Plates can move apart, move together, or slide past each other, and the type of movement creates a type of boundary.

Divergent boundaries (plates move apart)

At a divergent boundary, two plates move away from each other. The gap is filled by magma rising from the mantle, which cools to form new crust. This often happens under the ocean, building a long underwater mountain chain called a mid-ocean ridge. The hazards are mostly gentle volcanic eruptions and small earthquakes. An example is the Mid-Atlantic Ridge.

Convergent boundaries (plates move together)

At a convergent boundary, two plates push toward each other, and what happens depends on the crusts involved:

• Oceanic meets continental: the denser oceanic plate is forced down beneath the lighter continental plate. This is called subduction. The sinking plate melts, feeding violent volcanoes, and the crumpling forms fold mountains and powerful earthquakes.
• Continental meets continental: neither plate sinks easily, so they crumple upward into high fold mountains such as the Himalayas, with strong earthquakes but few volcanoes.

Transform boundaries (plates slide past)

At a transform boundary, two plates slide past each other in opposite directions or at different speeds. Their rough edges lock together, pressure builds, and when they suddenly jerk free the stored energy is released as an earthquake. There is no subduction and usually no volcano, but earthquakes can be severe. An example is the San Andreas Fault.

Examples in context

Example 1. The Sunda Trench off Indonesia. Off the coast of Sumatra and Java, the Indo-Australian Plate is subducting beneath the Eurasian Plate at a convergent boundary. This produces the deep Sunda Trench, the volcanoes of Indonesia, and some of the world's most powerful earthquakes, including the 2004 event that caused a major tsunami. It is a textbook oceanic-continental convergent boundary close to Singapore's region.

Example 2. The Himalayas as a continental collision. The Indian Plate is pushing into the Eurasian Plate, a convergent boundary between two continental plates. Because neither sinks easily, the crust crumples upward into the Himalayas, the world's highest mountains, with frequent strong earthquakes but few volcanoes. It shows how the type of crust changes what a convergent boundary produces.

Try this

Q1. Name the type of boundary where two plates move apart. [1 mark]

• Cue. A divergent boundary.

Q2. Explain why the oceanic plate sinks at an oceanic-continental convergent boundary. [2 marks]

• Cue. The oceanic plate is denser than the continental plate, so it is forced down beneath it in a process called subduction.

Q3. State one landform or hazard found at a convergent boundary and one found at a divergent boundary. [2 marks]

• Cue. Convergent: fold mountains, volcanoes or strong earthquakes. Divergent: a mid-ocean ridge with new crust and gentle eruptions.